The problem of trying to produce water-free oil from an oil reservoir in which water is an integral part of the environment is as old as the oil business. Although this problem occurs at any stage in the life of an oil well, from date of discovery to abandonment, it becomes increasingly vexatious with time and the decline of oil reserves in the field. Ultimately, when the lifting costs of the combined oil and water exceed the value of the recovered oil, abandonment becomes the only alternative. As production nears this stage, the local area of the oilfield is considered as being exhausted and the well is termed a "stripper".
Many procedures have been tried to produce water-free oil. As far as I know, the most widely used method involves emplacing a cement plug at the oil-water contact, perforating the casing above but near the oil-water contact to allow the oil to enter into the casing, and pumping from a position close to the cement plug to insure that there is a sufficient column of oil to pump from, since the oil column in stripper wells is typically quite thin. Another procedure sometimes used consists of injecting just above the oil-water contact, a "sheet" of cement into a large peripheral area through a ring of perforations in the casing with the objective of blocking off rising water. As pumping resumes, however, removal of oil from the water table permits the water to rise slowly in its place and hence the remedial step must be re-performed.
Yet another method has been tried without much success. An open-ended casing is run down to within about a foot above the top of the oil zone, without entering it. Then a pump is installed in the casing so its operation pulls oil up through the bottom of the casing. This method fails after some oil is produced because water comes up through and with the oil.
In most oil-water producing wells, with each suction stroke of the pump, as it draws fluids into the bore, sand and silt also flow into the well bore through the bottom of the well or through casting perforations. This problem is normally handled by "cementing off" the sands arriving through existing perforations, and reperforating the casing at some point above the cemented zones. As production decreases, the process is repeated.
To control such sand and silt inflows, many operators have employed the water well driller's technique of gravel packing an annular space around the perforated casing that straddles the oil saturated zone. This technique is successful in precluding sand and it allows the operator to pump fluid at higher rates, in ratios of up to 100 bbls. of water to 1 bbl. of oil, but here the water problem increases. However, there is some compensation to the operator in this method because the induced cone of depression in the water table affects a relatively larger area than the immediate well-bore and draws in oil from the entire area affected by this method. With this compensation comes a greater need for separating oil from water and disposing of the latter, usually brine. Thus fluid lifting costs increase and brine disposal also requires increasing expenditures.